Expert advice to ace container gardening this summer

Chicago Botanic Garden

When gardening with containers, be sure to buy a commercial, lightweight, soil-less growing medium designed for container gardening. If you put garden soil into a container, both aeration and drainage will be severely impeded.

When gardening with containers, be sure to buy a commercial, lightweight, soil-less growing medium designed for container gardening. If you put garden soil into a container, both aeration and drainage will be severely impeded. (Chicago Botanic Garden)

Tim JohnsonChicago Tribune

I need some advice on the best practices to use for gardening with containers. I want to add some color to my patio this summer.

— Debbie Johnson, Woodstock

Start by purchasing a high-quality growing medium for your containers. It is best to use a commercial, lightweight, soil-less growing medium that is designed for container gardening and not garden soil, even if you amend it with sand or compost. If you put garden soil into a container, both aeration and drainage will be severely impeded, no matter how good your soil is or what you amend it with.

Growing mixes for containers are always blended to ensure proper drainage and aeration. There is no need to put a layer of gravel at the bottom of your containers for drainage, though it is important that your containers have a drainage hole or holes in the bottom for excess water to run out. Unless your container is very small, the drainage hole should be around three-quarters of an inch in diameter or it will easily plug up. A screen or clay shard can be placed over the hole to prevent the medium from washing out.

Choose containers that are the right size to be in scale with the surrounding landscape. For instance, if you have a small apartment balcony, choose a smaller pot. Plants in smaller containers will need more frequent watering. Terra-cotta containers should be stored indoors in winter; if left outside, they will absorb water and crack. Fiberglass, stone, lead and concrete containers can typically be left outdoors in winter.

If your containers are large, consider filling some of the open space in the bottom with old plastic pots or a bag full of shipping peanuts to save money on the growing medium that you need to buy. I often use the plastic pots of the plants that I have purchased to fill in the bottom of larger containers.

Depending on what plants you plan to grow, you’ll need a foot or so of growing medium above the filler material. The bigger the plants that you want to grow, the more growing medium you need in the pot. Avoid filling your containers to the top — it will be easier to water the plants if the growing medium is slightly below the lip of the container. If the growing medium is too high, water will run over the sides of the container. For a spring container planting of low-growing violas, I slightly mounded the growing medium in the pot so that the flowers were more visible — that container needs to be watered carefully.

Apply a slow-release fertilizer when you plant the containers for the summer season. These fertilizers will gradually release nitrogen for plants to use. The more frequent watering of containers can leach nutrients out of the growing medium, so supplement with a liquid fertilizer a couple times during the summer if needed. Signs of “hungry” plants include reduced growth and an off-green leaf color.

On occasion, I reuse some of the previous year’s growing medium by replacing about half of the old medium with fresh medium. In that sort of scenario, adding supplemental fertilizer will more likely be necessary for the best plant performance. It is important to avoid overfertilizing the plants, though. Too much nitrogen can result in oversized plants with few flowers.

Tim Johnson is director of horticulture for the Chicago Botanic Garden in Glencoe.

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